I have the same issue
I've checked the Defaultfilename= and it's empty " " nothing I do will populate this field. Next I will try hard coding.
// WA_UploadResult1_1 Start
$WA_UploadResult1_Params["WA_UploadResult1_1"] = array(
'UploadFolder' => "../pub/",
'FileName' => "[FileName]",
'DefaultFileName' => "",
'ResizeType' => "0",
'ResizeWidth' => "0",
'ResizeHeight' => "0",
'ResizeFillColor' => "" );
// WA_UploadResult1_1 End
// WA_UploadResult1 Params End
?>